The hepadnaviruses are a group of hepatotropic DNA viruses which produce acute and chronic infections of the liver and which replicate their genomes by reverse transcription of RNA intermediates. The principal objectives of this program are to elucidate the molecular mechanisms of hepadnavirus gene expression and genome replication. To achieve these objectives we will employ two general strategies: (1) biochemical analysis of viral RNA and DNA synthesis in cultured cells transfected with cloned viral DNA, and (2) genetic and biochemical analysis of the replication of mutant viral genomes following in vivo transfection of intact animal hosts. Using these systems, we propose to (1) screen existing cultured cell lines for permissivity for the growth of animal hepadnaviruses, (2) develop new strategies for the selection of such permissive lines, (3) examine and define the cis- and trans- acting components of the viral transcriptional program, (4) define the signals which specify the correct processing and packaging of viral RNA's, (5) explore the functions of the viral direct repeat (DR) sequences and other cis-acting elements in viral genome replication, and (6) clarify the strategies employed in the expression of the viral pol and X gene products. We will also attempt to use this improved understanding of the hepadnavirus life cycle to explore the possibility of generating replication- competent hepadnaviruses which can deliver foreign DNA sequences to liver cells.